1. Field of the Invention
This invention generally relates to a method of detecting sync signals. This invention particularly relates to a method of reproducing sync signals with different patterns from a recording medium, and generating a data-reading timing signal in response to the reproduced sync signals.
2. Description of the Related Art
Some optical discs such as a CD-R (Compact Disc-Recordable) and a CD-RW (Compact Disc-Rewritable) have a track formed by a groove which wobbles at a spatial frequency corresponding to 22.05 kHz. The wobble is the recording of information (wobbling information) of different types which contains 22.05-kHz information and phase-modulated address information representing the absolute position of every point on the track. The address information is called ATIP (absolute time in pre-groove). The wobbling information can be used as tracking and timing information.
A drive apparatus for such an optical disc includes an optical pickup which applies a spot of light to a track on the optical disc. The applied light is reflected at the surface of the optical disc, and there occurs reflected light inclusive of wobbling information. A portion of the reflected light returns to the optical pickup. The optical pickup converts the return light into a corresponding electric signal. The drive apparatus detects the wobbling information from the electric signal generated by the optical pickup. During a recording or playback mode of operation, the drive apparatus uses the detected wobbling information as a signal for controlling the rotation of the optical disc and also a signal for generating a reference clock signal.
Other optical discs such as a DVD-R (Digital Versatile Disc-Recordable) and a DVD-RW (Digital Versatile Disc-Rewritable) have a track with a wobble recorded on a non-modulation basis and designed to provide 140-kHz clock information. In these optical discs, address information is previously recorded as pre-pits in a land portion between adjacent groove portions.
In the case of an optical disc in which address information and a sync signal for detecting the address information are represented by a phase-modulated wobble of a track, a first example is designed so that “0” and “1” in the address information are expressed through inversion of the phase of wobbling information. In a second example, “0” and “1” in the address information and the sync signal are expressed by different patterns resulting from combining basic wobbles of different phases. The modulated phase information is recovered by subjecting reproduced wobbling information to synchronous detection in response to a clock signal extracted from the reproduced wobbling information.
Generally, address information recorded on an optical disc is based on time sharing, and accurate detection of a sync signal preceding the address information is required for correct recovery of the address information.
Japanese patent application publication number 5-151708/1993 discloses a synchronization protecting device designed to compensate for synchronization discontinuities caused by random code errors and burst errors, and to prevent false synchronization due to pseudo synchronization patterns and to rapidly restore synchronization right after a head switching. The device of Japanese application 5-151708/1993 has a structure for accurately detecting a sync signal pattern.
Specifically, the device of Japanese application 5-151708/1993 includes a sync pattern detection circuit, a first sync protection circuit, and a second sync protection circuit. The sync pattern detection circuit compares a prescribed number of current successive bits in an input digital signal with bits of a true sync signal pattern. When all the current successive bits in the input digital signal match with the bits of the true sync signal pattern, the sync pattern detection circuit outputs a sync pattern detection signal “A”. When only one of the current successive bits differs from corresponding one of the bits of the true sync signal pattern, the sync pattern detection circuit outputs a sync pattern detection signal “B”. The first sync protection circuit decides whether or not the sync pattern detection signal “A” is repetitively outputted at a correct period, that is, whether or not the synchronization is established. This decision is carried out by comparing the sync pattern detection signal “A” with a repetitively-generated internal sync signal. In the case where the synchronization goes unestablished and is thereafter re-established, the first sync protection circuit outputs a first pulse signal in response to the re-establishment of the synchronization. The second sync protection circuit decides whether or not the sync pattern detection signal “B” is repetitively outputted at a correct period. This decision is carried out by comparing the sync pattern detection signal “B” with a repetitively-generated internal sync signal. While the sync pattern detection signal “B” remains repetitively outputted at the correct period, the second sync protection circuit repetitively outputs a second pulse signal at the same period as a detection-result sync signal. When the first pulse signal is outputted from the first sync protection circuit, the second sync protection circuit outputs a detection-result sync signal synchronized with the first pulse signal.
As the information recording density of an optical disc increases, the track pitch thereof decreases. A smaller track pitch causes a drop in S/N (signal-to-noise ratio) or C/N (carrier-to-noise ratio) regarding the detection of a timing signal or a sync signal from a wobble of a track on the optical disc. In addition, a smaller track pitch increases cross talk between wobbles of adjacent track portions regarding the reproduction of information from the optical disc. -The increased cross talk deteriorates reproduced wobbling information, and causes false inversion of the phase of the reproduced wobbling information. As a result of these factors, there is a chance that a reproduced sync signal will have a bit shift from the original and that the accuracy of the detection of a sync signal will drop to an unacceptable range and hence the correct recovery of the wobbling information in response to the detected sync signal will be unfeasible. Basically, these problems can not be solved by the above-mentioned device of Japanese application 5-151708/1993. Similar problems may occur due to degradation of the optical disc.